1. Field of the Invention
This invention relates to a cavity and filter containing said cavity and to a method of constructing said cavity with one or more irises containing eccentric circular apertures.
2. Description of the Prior Art
It is known to couple energy between cylindrically shaped cavities using a circular aperture located in a cross wall separating adjacent cavities. In FIGS. 2 and 2(a), (see U.S. Pat. No. 4,652,844, naming Brambilla as inventor), there is described, as prior art, two cavities separated by a cross wall Pti, which contains a centrally located circular opening Ai. The Brambilla Patent describes an arcuate aperture for use in conjuntion with an adjusting screw for coupling between adjacent cavities.
U.S. Pat. No. 4,030,051, naming Shimizu et al as inventor, describes a microwave resonator having a rotary joint for variable coupling between cavities. The rotary joint is located at the midpoint of a cavity and apertures, having an elliptical shape, are centered in an iris plate. The patent states that coupling into and out of the cavity may be accurately varied simply by rotating the portion of waveguide on opposite sides of the rotating joint relative to one another.
A prior art cylindrical cavity structure is shown in FIG. 1 where a filter 2 has two cavities 4,6 separated by an iris 8 having a centrally located cruciform aperture 10. The filter has an input 14 and an output 16 and each cavity has 3 tuning screws 18 to provide the desired coupling and phase balance. The arrangement of the tuning screws is shown in FIG. 2(a), which represents a prior art schematic end view of the tuning screws 18 in one of the cavities 6. From FIG. 1, it can be seen that the tuning screws 18 in the cavity 4 are oriented in a different arrangement than the arrangement of the tuning screws 18 in cavity 6.
In another prior art embodiment shown in FIG. 2(b), the tuning screws 18 are replaced by short rectangular posts 20 in cavity 6 (see Guglielmi et al, "Dual-mode Circular Waveguide Filters Without Tuning Screws", IEEE, Microwave Guided Wave Lett., VOL. 2, pages 457 to 458, Nov. 1992 and Beyer et al, "Efficient Modal Analysis of Waveguide Filters Including The Orthoginal Mode Coupling Elements by a MM/FE Method", IEEE Microwave Guided Wave Lett., VOL. 5, Jan. 1995). It should be noted that the rectangular posts vary in size from one another. The structure is analyzed using a pure numerical Finite Element Method (FEM) analysis. Rectangular posts 22 shown in cavity 6 in prior art FIG. 2(c) have been modified to make the analysis easier (see Vahldieck, "A Combined Mode Matching/Method of Lines Approach For Field-theory Analysis Of Dual Mode Filters", Proceedings of ESA Workshop in Advanced CAD for Microwave Filters and Passive Devices, pages 1 to 15, Nov. 1995).
In Accatino et al., "A Four-pole Dual Mode Eliptic Filter Realized in Circular Cavity Without Screws", IEEE Trans. Microwave Theory Tech., VOL. MTT-44, pages 2680-2687, Dec. 1996, as shown in FIG. 2(d), the cavity 6 has an iris 24 having a rectangular aperture 26. The iris is located in the middle of the resonant cavities and coupling and tuning mechanisms are obtained by rotation angle of the rectangular aperture and by size of the rectangular aperture relative to the size and thickness of the iris sections. The prior art arrangement shown in FIG. 2(d) has several advantages over previous structures. Unfortunately, the structure shown in FIG. 2(d) suffers from disadvantages as well. For example, in order to construct the irises containing the rectangular apertures, sophisticated mechanical processes are required, for example, electro-discharge machining to ensure that the corners of the rectanglar aperture are sharp. Further, the minimum ratio of remaining conductor surface area over the cavity cross section is as large as (.pi.-2)/.pi.. This results in the conductor loss on the remaining surface being large, which in turn decreases the unloaded Q of the filter. An iris of a small aperture in a TE.sub.11n mode circular cavity may increase a risk of having spurious modes in the frequency band of interest. FIG. 2(e) describes a cavity 6 having an iris 27 with elliptical aperture 29.